Single-Molecule Junctions: Vibrational and Magnetic Degrees of Freedom, and Novel Experimental Techniques

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1 Single-Molecule Junctions: Vibrational and Magnetic Degrees of Freedom, and Novel Experimental Techniques Heiko B. Weber Lehrstuhl für Angewandte Physik Friedrich-Alexander-Universität Erlangen-Nürnberg 1

2 Chair for Applied Physics Electronic Properties of epitaxial graphene Electron transport on the molecular scale Semiconductors and their doping (Dr. Michael Krieger) THz generation and detection (Dr. Sascha Preu) 2

3 People Ferdinand Kisslinger Benjamin Gmeiner Daniel Secker Stefan Wagner Stefan Ballmann Konrad Ullmann 3

4 How does charge flow through single molecules? - Introduction: our technique - vibrational degrees of freedom - magnetic degrees of freedom - novel experimental tools 4

5 How does charge flow through single molecules? - Introduction: our technique - vibrational degrees of freedom - magnetic degrees of freedom - novel experimental tools 5

6 Single - molecule contacts nm Mechanically controlled break junction technique 6

7 Experimental Mounted either in a 3 He or 4 He cryostat, in vacuum, at temperatures down to 300mK 7

8 Conductance traces 8

9 First experiments Symmetric molecule Asymmetric molecule 2 nm with Marcel Mayor, Phys. Rev. Lett. 88, (2002) 10

10 Room temperature vs. low temperature T=300 K Phys. Rev. Lett. 88, (2002) Appl. Phys. Lett. 82, 4137 (2003) T=30 K 11

11 Room temperature vs. low temperature T=300 K What determines the current level? What is the nature of the first peak? T=30 K 12

12 How does charge flow through single molecules? - Introduction: our technique - vibrational degrees of freedom - magnetic degrees of freedom - novel experimental tools 13

13 Vibrations Electronic processes in molecules are related to vibrations! Electron transfer Optical absorption 14

14 Simplest model One electronic level ε and one vibrational mode ω: 15

15 Comparison with quantum dots We have fixed gate potential, provided by the local environment O S C 12 H 15 O C 12 H 15 S V SD [mv] H. Van der Zant et al. V G [V] 16

16 Peaks appear too broad For nearly all molecules: At low voltages vibrations may be resolved, at higher V peaks appear smeared Seeking for a model system for vibrations 17

17 Single-molecule string CH 3 CH 3 H 3 C CH 3 H 3 C CH 3 P P Au S Pt C C C C C C C C Pt S Au P P H 3 C CH 3 H 3 C CH 3 John Gladysz, Erlangen / Texas A&M CH 3 CH 3 Oligoyne: ideal linear sp bond - no aryl rings, no rotational degree of freedom - heavy Pt ions provide fixed ends ChemPhysChem 11, 2256 (2010) 18

18 Density functional theory DFT (calculations using TM 5.7: BP86/SVP,TZVP, B3LYP/TZVP Electronic structure + vibrational frequencies (3N-6 normal modes) DFT in collaboration with: Wolfgang Hieringer, Andreas Görling, Erlangen 19

19 Vibrational features Hypothesis: longitudinal modes DFT (longitudinal string-like excitations) Experiment 37.9 mv 37 ± 1.5mV 79.4 mv 89 ± 0.9mV mv 130 ±1.5 mv Vibrations are detected as sidepeaks, but the peaks are still too broad! 20

20 Further oligoines Martin Bryce, Durham vibrational energies [mev] longitudinal modes

21 Theory: vibrations and decoherence In dithiols: nearly degenerate orbitals, which differ in symmetry 28

22 Vibrations Theory: destructive interference, suppression of conductance, lifted by vibrations 29

23 Experiment Experiment: stability check 30

24 Data extraction 31

25 Broken symmetry Experiment 32

26 Split conjugation longitudinal modes 33

27 A counter example Experiment Martin Bryce, Durham When even/odd state pairs are remote, no T dependence is observed. 34

28 Vibrations and Interference Intermediate conclusion: Oligoines show vibrational substructure in the first peak Destructive interference provided by degenerate levels suppresses conductance Vibrations contribute to enhance the current level D. Secker, S. Wagner, S. Ballmann, R. Härtle, M. Thoss, H.B. Weber, PRL 106, (2011) S. Ballmann, R. Härtle, P.B. Coto, M. Elbing, M. Mayor, M.R. Bryce, M. Thoss, H.B. Weber, PRL 109, (2012) 35

29 How does charge flow through single molecules? - Introduction: our technique - vibrational degrees of freedom - magnetic degrees of freedom - novel experimental tools 36

30 Molecular Spintronics Even electron number: Can we detect the spin state of a coupled spin pair? A spin pair occurs either as singlet or triplet Here: pseudo singlet, pseudo triplet (Karin Fink, KIT) Transition from S=0 to S=2 Mario Ruben, Frank Schramm (KIT) 37

31 Molecular Spintronics Transition from S=0 to S=2 Black: pseudo triplet Red: pseudo singlet Karin Fink (KIT) 38

32 Blind experiments Transition from S=0 to S=2 39

33 Kondo effect Kondo effect in quantum dots: Zero bias conductance enhancement due to a new correlated ground state in which an unpaired spin is screened by the surrounding electrons Jun Kondo 40

34 Kondo-like anomaly Transition from S=0 to S=2 41

35 Magnetic degree of freedom Transition from S=0 to S=2 42

36 Switching the spin states Transition from S=0 to S=2 43

37 Switching the spin states Transition from S=0 to S=2 44

38 Magnetic degrees of freedom Intermediate conclusion: The results suggests that one observes the spin state of a pair of a coupled spin system by a Kondo-like anomaly We observe a bias-driven transition from the pseudo-singlet to the pseudo-triplet state S. Wagner, F. Kißlinger, S. Ballmann, F. Schramm, R. Chandrasekar, T. Bodenstein, O. Fuhr, D. Secker, K. Fink, M. Ruben, H. B. Weber: Nature Nanotechnology (2013) longitudinal modes 45

39 How does charge flow through single molecules? - Introduction: our technique - vibrational degrees of freedom - magnetic degrees of freedom - novel experimental tools SFB 953: Synthetic carbon allotropes 46

single-molecule contacts -Gate action is

40 Experimental improvements Stacked, gateable break junction: Synthesis: Frank Schramm, Mario Ruben Karlsruhe Nearly ideal device achieved - 9 nm low-κ dielectric (Ta 2 O 5, ε = 27) - <10 pa leakage currents at 7V - single-molecule contacts -Gate action is still poor and inhomogeneous New Journal of Physics 14, (2012) 47

41 SFB 953: Synthetic carbon allotropes Synthesis: Frank Schramm, Mario Ruben Karlsruhe 48

42 Epitaxial graphene We produce epitaxial monolayer graphene on wafer scale on silicon carbide K. Emtsev H.B.Weber, T. Seyller: Nature materials 8, 203 (2009) Nature Materials 8, (2009) 49

43 Scheme Creation of graphene-electrodes with nanometer separation 50

44 Scheme Goal: creation of graphene-electrodes with nanometer separation Contacting single molecules 51

45 Fabrication of nanogaps Sample preparation: 53

46 data (unpublished data are removed in this public version) 54

gates STM, AFM, light pulses Raman spectroscopy High-frequency

47 Why graphene junctions? Molecular Nanojunctions become accessible with/to electrostatic gates STM, AFM, light pulses Raman spectroscopy High-frequency exitations chemistry superconductivity D. Waldmann H.B.W. Nature materials. 10, 357(2011) THz antenna 55

48 Conclusions Charge transport in single molecules is affected by vibrations - the first peak - vibrations lift interferences Magnetic moments are detectable as Kondo anomalies, - the combined spin of two magnetic cores can be detected New tools are available: - gated break junctions - graphene electrodes 56

A & M Mario Ruben, Karlsruhe Nazario Martin, Madrid Martin Bryce, Durham Rik Tykwinski, Erlangen

49 People In my group: Daniel Secker Stefan Ballmann Synthesis: Frank Schramm, Mario Ruben Karlsruhe Stefan Wagner Konrad Ullmann Ferdinand Kisslinger Chemistry: Marcel Mayor, Basel John Gladysz, Texas A & M Mario Ruben, Karlsruhe Nazario Martin, Madrid Martin Bryce, Durham Rik Tykwinski, Erlangen Theory: Andreas Görling, Erlangen Felix von Oppen, Berlin Michael Thoss, Erlangen Karin Fink, Karlsruhe 57

Single-Molecule Junctions with Epitaxial. Graphene Nanoelectrodes. (Supporting Information)

Single-Molecule Junctions with Epitaxial Graphene Nanoelectrodes (Supporting Information) Konrad Ullmann, Pedro B. Coto, Susanne Leitherer, Agustín Molina-Ontoria,, Nazario Martín, Michael Thoss, and Heiko